In one known form of steerable drilling system, a motor is connected to and carried by a drill string, the motor being designed such that an output shaft thereof is angled to the axis of the associated end part of the drill string. A drill bit is connected to the motor so as to be driven for rotation thereby.
Progressive cavity pumps or motors, also referred to as a progressing cavity pumps or motors, typically include a power section consisting of a rotor with a profiled helical outer surface disposed within a stator with a profiled helical inner surface. The rotor and stator of a progressive cavity apparatus operate according to the Moineau principle, originally disclosed in U.S. Pat. No. 1,892,217.
In use as a pump, relative rotation is provided between the stator and rotor by any means known in the art, and a portion of the profiled helical outer surface of the rotor engages the profiled helical inner surface of the stator to form a sealed chamber or cavity. As the rotor turns eccentrically within the stator, the cavity progresses axially to move any fluid present in the cavity.
In use as a motor, a fluid source is provided to the cavities formed between the rotor and stator. The pressure of the fluid causes the cavity to progress and a relative rotation between the stator and rotor. In this manner fluidic energy can be converted into mechanical energy.
As progressive cavity pumps or motors rely on a seal between the stator and rotor surfaces, one of or both of these surfaces preferably includes a resilient or dimensionally forgiving material. Typically, the resilient material has been a relatively thin layer of elastomer disposed in the interior surface of the stator. A stator with a thin elastomeric layer is typically referred to as thin wall or even wall design.
An elastomeric lined stator with a uniform or even thickness elastomeric layer has previously been disclosed in U.S. Pat. No. 3,084,631 on “Helical Gear Pump with Stator Compression”. The prior art has evolved around the principle of injecting an elastomer into a relatively narrow void between a stator body with a profiled helical bore and a core, or mandrel, with a profiled helical outer surface. The core is then removed after curing of the elastomer and the remaining assembly forms the elastomeric lined stator. The elastomer layer is essentially the last component formed.
The stator bodies mentioned above have a pre-formed profiled helical bore. The profiled helical bore is generally manufactured by methods such as rolling, swaging, or spray forming, as described in U.S. Pat. No. 6,543,132 on “Methods of Making Mud Motors”, incorporated by reference herein. Similarly, a profiled helical bore can be formed by metal extrusion, as described in U.S. Pat. No. 6,568,076 on “Internally Profiled Stator Tube”, incorporated by reference herein. Further, various hot or cold metal forming techniques, such as pilgering, flow forming, or hydraulic forming, as described in P.C.T. Pub. No. WO 2004/036043 A1 on “Stators of a Moineau-Pump”, incorporated by reference herein, can be used to form a stator body with a profiled helical bore.
A stator body can also be formed by creating a profited helical bore in relatively thin metal tubing. This formed metal tube can then be used as the stator body by itself, with an injected inner elastomeric layer, or the formed metal tube can be inserted inside into a second body with a longitudinal bore to form the stator body. A stator body with a profiled helical bore can also be formed through other process such as sintering or hot isostatic pressing of powdered materials, for example, a metal, or the profiled helical bore can be machined directly into a body.
In use, the motor is driven to rotate the bit, and a load is applied to the bit. As a result, the bit scrapes, abrades or gouges material from the formation being drilled. Where it is required to drill straight ahead, the drill string is rotated so that the direction in which the drill bit is pointed constantly changes, precessing around the desired drilling direction. To form a curve in the borehole, rotation of the drill string is halted with the motor orientated such that the drill bit tool face is directed in the desired direction.
Stopping rotation of the drill bit in this manner is undesirable as there is the risk of differential sticking, particularly in depleted zones. Further, continued drilling with the drill string non-rotating requires the drill string to slide within the borehole, reducing the weight-on-bit load which can be applied to the bit and thus slowing drilling.
Further disadvantages with this type of system are that stopping the drill string with the bit pointing in the desired direction is difficult, and that once this has been achieved, operation of the motor results in the application of a reactive force which can result in the motor shifting to an angular position in which the bit is no longer pointing in the desired direction. Time must then be spent adjusting the angular position of the drill string to move the motor back to the desired orientation.
It is an object of the invention to provide a drilling system in which these disadvantages are of reduced effect.